[devices] Use GuestMemory trait instead of the GuestMemoryMmap

[andreeaflorescu]

This has a slight overlap with https://github.com/firecracker-microvm/firecracker/issues/1544. Created as a separate issue as it is needed when consuming devices from rust-vmm.

We should use the trait instead of the mmap implementation here. Better yet, devices should not depend on vm-memory's mmap backend feature. That feature should only be used as a dev-dependency.

[alexandruag]

Maybe it's a good idea to wait for discussions around topics such as this one to settle before proceeding with a particular course of action here, if that makes sense.

[roypat]

For now, we are not planning to do this, as we do not see benefits from this as a stand-alone change. Should a change like this become necessary for any specific task in the future, we can track doing it as part of that task.